1. Field of the Invention
The present invention relates to the art of opening blast furnace tap holes and to blast furnace tap hole drills, and more particularly to new and improved extension bar as typically utilized within drill rod assemblies on such tap hole drills. The extension bar of this invention will have a longer useful life and minimize the tendency for it to become welded to the drill rod, or in particular being partially destroyed by molten metal, thereby facilitating its easy removal to further extend the average useful life of the drill rod.
2. Summary of the Prior Art
It is well known that the hearth of an iron blast furnace is provided with a tap hole, commonly referred to as a "iron notch", through which molten iron, usually referred to as "hot metal", is drawn off at periodic intervals during the blast furnace campaign. During a normal campaign, such tapping must be done on an average of five to twelve times daily, as the blast furnace hearth becomes filled with molten iron and slag. After the blast furnace has been tapped, i.e. the molten hot metal and slag drained therefrom, the tap hole or iron notch is plugged with clay or "mud" which will harden and seal the tap hole until the next time the blast furnace is tapped.
In accordance with usual practices, a special drill is utilized to open a tap hole, i.e. drill a passageway through the hardened clay plugging the iron notch, for the purposes of tapping the blast furnace. Such blast furnace tap hole drills are normally pneumatically or hydraulically operated rotary percussion drills comparable to the rock drills utilized in the mining industry. Such drills impart both a rotary and an impact force on an elongated drill rod having a rock drill bit at the end toward the iron notch.
The base support for the blast furnace tap hole drill is normally secured to the floor, a structural column or some solid base structure, and is provided with suitable linkage members and remote controls so that the blast furnace tap hole drill can be remotely operated to move the drill into the proper position for drilling the tap hole, then operated to drill the tap hole, and thereafter, moved back away from the tap hole and heat of the emerging hot metal, where the drill can be serviced and prepared for the next tap.
To prepare the blast furnace tap hole drill for each succeeding tap, it is always necessary to replace the drill bit, if not the drill rod or a portion thereof. This is because the temperature of the blast furnace hot metal, being about 2700-2800.degree. F., severely erodes the drill bit after it drills through the clay plug and enters the bath of molten hot metal. In addition, once the tap hole is drilled, the ferrostatic head of hot metal within the blast furnace will cause the hot metal to emerge through the tap hole around the drill bit and drill rod before the drill rod and bit can be withdrawn from the tap hole. Often times, the drill bit will not only be severely eroded but the portion remaining will virtually be "welded" to the end of the drill rod to which it had previously been removably attached. In such an event, it will be impossible to remove the drill bit from the drill rod to replace a new drill bit, and accordingly it then becomes necessary to replace the entire drill bit and adjoining drill rod or drill rod component to which it is welded.
To reduce the expense of replacing the entire drill rod and bit assembly, it has become common practice to utilize a drill rod extension bar, which is merely a removable portion of the drill rod, typically from 18 to 30 inches in length, fitted between the elongated rearward portion of the drill rod and the drill bit. Therefore, when the drill bit becomes welded to the extension bar, or the extension bar otherwise damaged, the bit and extension bar can be replaced without the need for replacing the elongated drill rod or entire drill rod assembly.
In utilizing and extension bar, however, it still at times happens that the outward pouring of hot metal around the extension bar, as above noted, will cause severe erosion at the interface between the extension bar and drill rod, often times welding even these two components together. In such an event, it will still be necessary to replace the entire drill rod assembly. Even though the drill rod, extension bar and coupling are sometimes provided with a protective ceramic coating, the hot metal often attacks, erodes and welds the uncoated contacting interfaces between the coupling and extension bar or drill rod, just as it attacks the interface between the drill bit and extension bar.
While it is of course possible to remove the drill bit from the extension bar or the extension bar from the drill rod with an acetylene cutting torch, use of this procedure will usually damage the threaded ends of the extension bar or drill rod so severely that they cannot be reused anyway, so that nothing useful can be salvaged by using this technique.